Fluorescent display devices, used in calculators and other logic-driven systems, utilize phosphor-covered electrodes which are excited into optical emission by the impingement of thermoelectrons emitted by a low temperature electrically heated filament in a vacuum or by electrically excited ions in a gaseous environment. The electrodes are usually mounted upon a flat glass or ceramic substrate and the electrodes and filament are enclosed in a concave cover, usually of glass. The cover typically has a peripheral flange adapted to face-to-face mating with the substrate and is usually hermetically sealed thereto using a low-temperature frit forming a sealed enclosure between the cover and the substrate. The sealed enclosure is conventionally evacuated or gas filled, tipped off and gettered by means well known in the art.
At the low interelectrode accelerating voltages typical in fluorescent display devices, external and internal electrostatic fields can have a significant and varying effect on the illumination of the phosphor coated electrodes. This effect was recognized and countered in U.S. Pat. No. 3,584,252 using a screen around part of the circumference inside a vacuum tube. U.S. Pat. Nos. 4,004,186 and 3,668,466 teach the use of a layer of transparent conductive coating inside the transparent cover plate to shield the interior of the fluorescent display device from external electrostatic fields.
The transparent conductive coating using, for example, SnO or InO, is applied to the inside of the cover plate by vapor deposition, chemical deposition or sputtering.
Heretofore, in order to obtain a reliable seal between the flange on the cover plate and the substrate, it was universally believed that the flange on the cover plate which mated with the substrate could not be coated with the transparent conductive coating. Consequently, expensive masking or removal procedures were required to either protect the flange area from being coated or to remove the coating from this region after coating.
In some cases, such as shown in the referenced patents, it is desirable to make electrical contact between external circuits and the conductive coating. This has conventionally been accomplished by providing a metallic conductor through the hermetic seal and a contact piece, usually resiliently held in mechanical and electrical contact with the conductive coating, within the sealed enclosure.
A prevalent failure mode of fluorescent display devices of the type described in the preceding occurs at the peripheral seal in the vicinity of the metallic conductors piercing the seal. These failures may be discovered immediately upon manufacture, which increases scrap at the most costly point in the assembly sequence, or it may develop in use, which damages the reputation of the manufacturer as well as increasing the cost of a warranty program.
Hermetic seal failures are approximately proportional to the number of metallic conductors piercing the seal. It is therefore desirable to reduce the number of metallic conductors piercing the seal as much as possible.